The present invention relates generally to atomic absorption spectroscopy and more particularly to a furnace for electrothermal atomization of samples in atomic absorption spectroscopy.
Electrothermal atomizers, commonly referred to as heated graphite atomizers or graphite furnaces, are utilized in atomic absorption spectrophotometers for rendering the sample to be analyzed into atomic form. Typically, the furnace comprises a tubular graphite member clamped between annular graphite contacts or electrodes engaging its respective ends. A radial aperture in the side wall of the tubular member at the mid-point of its length serves as a sample port accommodating the insertion of the substance to be analyzed into the tubular member.
The contacts, usually mounted in cooling jackets, are pressed into tight engagement with the ends of the tubular furnace member by resilient biasing means or a servomotor. An intense electrical current is passed longitudinally through the tubular member between the contacts and heats the member to the high temperature required to convert the sample to a "cloud of atoms".
A measuring light beam of a line emitting light source which comprises the resonant spectral line of a looked-for element is passed through the annular graphite contacts and the longitudinal bore of the graphite tube. The amount of the looked-for element in the sample can be determined from the absorption of the measuring light beam.
In order to prevent rapid deterioration of the tubular graphite member by oxidation at the high temperatures required for atomization of the analyte, provision is made for enveloping it in a flow of inert protective gas. The graphite tube is surrounded by the inert gas such that oxygen does not come into contact with the graphite tube.
A non-uniform temperature distribution along the graphite tube results when the graphite tube is held at its ends. The graphite tube has a higher temperature at its central area than at the ends where the heat dissipates to the cooler contacts. This non-uniformity of temperature results in the deposition of sample on the cooler ends of the tubular member; the deposit is re-evaporated in subsequent use of the tubular member thereby contaminating the new sample.
A graphite furnace of the type just described is shown in Braun et al., U.S. Pat. No. 4,022,530 which is incorporated herein by reference. In this particular furnace, the contacts are tubular rather than annular. The two contacts extend around the graphite tube along its entire length between the contact surfaces except for a separating gap. An inert gas flow is passed into the graphite tube from both ends. This inert gas flow emerges through a radial bore of the graphite tube in its center. One of the tubular contacts has a radial bore which is aligned with the radial bore of the graphite tube.
In an attempt to achieve a more advantageous temperature distribution along the graphite tube, it has been proposed to pass the heating current transversely through the graphite tube rather than longitudinally. For this purpose, a contact arrangement is described in Woodriff, U.S. Pat. No. 4,407,582 wherein two pairs of interconnected contacts in the form of fork-shaped contact pieces are employed which engage the graphite tube radially on opposite sides. The heating current flows in a circumferential direction through the graphite tube in the area of the ends. The graphite tube is heated in the area of its ends and heat flows from the ends to the center to obtain a more uniform temperature distribution.
In this known contact arrangement, the electrodes engage the hot parts of the graphite tube; consequently, the reproducibility of the contact characteristics is poor. Furthermore, it is difficult to protect the graphite tube from exposure to atmospheric oxygen by means of an inert protective gas flow, resulting in short useful life of the graphite tube.
In Hutsch et al., U.S. Pat. No. 4,726,678 which is incorporated herein by reference and the publication in "Analytical Chemistry" 58 (1986), 1973, a graphite furnace is described in which the tubular furnace body has a rectangular cross-section and contact projections extend transversely to the axis of the furnace body. The furnace body and contact projections are formed as one integral graphite element. The contact is established in a cold zone by planar contact surfaces.
In some embodiments of U.S. Pat. No. 4,726,678, the contact projections between the contact surfaces and furnace body have areas of a reduced cross-section. In one embodiment (FIG. 3), cutouts are provided which extend longitudinally to the furnace body parallel to the furnace axis. These slots or cutouts provide a decrease of the heat flow from the furnace body to the ends of the contact projections and match electrical resistance to the output of the electrical power supply. In another embodiment (FIG. 4), it is stated that the contact projections are provided with multiple apertures and that this configuration is specifically suited for setting predetermined temperature profiles. Current is supplied at certain locations and flows through the furnace body in order to generate Joul's heat. This is similar to the arrangement of U.S. Pat. No. 4,407,582, except that the contacting is displaced to a cooler area.
In the known arrangement, the power is supplied nonuniformly along the tubular furnace body.
It is an object of the present invention to provide a new and improved furnace for electrothermal atomization.
Another object of the invention is to provide an electrothermal atomization furnace which attains uniform temperature distribution along the tubular furnace body.
A further object of the invention is to provide such a furnace which reduces heat dissipation of the tubular furnace body.
A further object of the invention is to provide such a furnace which facilitates effective protection against exposure to atmospheric oxygen.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
Accordingly, it has been found that the foregoing and related objects are attained in an electrothermal atomization furnace comprising a tubular furnace member adapted for passing a light beam therethrough and a pair of contact projections integrally formed longitudinally along said furnace member on opposite sides thereof so as to uniformly supply current along the furnace member and thereby produce uniform temperature distribution along the furnace member. The contact projections project outwardly from the furnace member with contact surfaces at the distal ends. The contact surfaces are adapted for supporting the furnace member between cooperating current-supplying contact elements. The contact projections have a central axis transverse to the furnace member and each contact projection has a contraction in cross-sectional area about the central axis which is configured and dimensioned to reduce heat dissipation from the furnace member and to increase the relative temperature of the contact projection at the contraction during sample atomization. The contraction is formed by cylindrical recesses on the upper and lower surfaces of the contact projection.
A substantially uniform current supply to the furnace body throughout the length of the furnace body is attained such that the furnace body is heated uniformly. The contractions of the contact projections counteract heat dissipation. On one hand, the cross-sectional area for heat dissipation is reduced. On the other hand, increased Joul's heat is generated in the region of the contraction which acts on the reduced mass such that the contraction assumes an increased temperature which reduces the dissipation of heat from the furnace body to the contact surface. Consequently, a very uniform temperature distribution results at the furnace body with relatively cold contact surfaces.